Forgive me: why sharpening is called 'unsharp masking'?


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maddog

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Feb 13, 2002
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Alexandra
maddog.clubsnap.org
#2
USM masks the unsharp characteristics of the image. Its a misnomer. USM increases the edge contrast in an image to make them more distinct.
 

Y

yjlim

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#3
Unsharp masking gets its name from a traditional darkroom sharpening technique with film. Basically this technique of sharpening uses both a blurry copy of the original photo (or "unsharp" copy) with the original negative to create a "mask" which is used to make another copy of the picture using a doubled exposure time. The unsharp mask negative and lengthened exposure time will then darken the dark side of an edge and lighten the light side, which causes the edge to display a halo and makes the edge more pronounced and sharpened.

The unsharp mask filter in programs work similar as they look for edges in pictures by examining the contrast and try to make the edges more pronounced. Yes, this filter has a bad name as it just confuses people. I remembered when I looked at the unsharp mask filter for the first time in photoshop and wondered by what the difference between blur and unsharpening was? ;)

for more info on unsharp mask:
http://www.pixilver.com/tutor_usm.html
 

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